A year ago, Dropbox was worth $4 billion. And Airbnb was worth $1.3 billion. So that's a 42 percent jump in their valuations—at least according to one of their most prominent shareholders. (Y Combinator's stake in companies varies, but it's usually around 7 percent.)

As far as we know, the companies haven't raised money since their latest megarounds. So what's Graham going on?

Another possible source of valuation figures are the 409a valuations companies must do when they issue stock options to employees, though for abstruse legal and financial reasons, those numbers can often come out lower than a company's most recent venture-financing valuation.